Low pressure gas discharge device with parallel electrodes and a sliding spark triggering electrode



Jan.30, 1968 A. MICHEL 3,366,824

LOW PRESSURE .GAS DISCHARGE DEVICE WITH PARALLEL ELECTRODES AND A SIJIDING SPARK'TRIGGERING ELECTRODE Filed Oct. 24, 1965 3 I 2 Sheets$heet 1 w v r g v v f; F lG.2

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v H; 4 k /4" v v DISCHARGESPACEI Y 2 ELECTRODE SURFACE DISC 5 ANODE4 Jan. 30, 1968 LOW PRESSURE GAS AND A SLIDING SPARK TRIGGERING ELECTRODE 24; 1965 2 Sheets-Sheet "2 Filed Oct.

A G a u I n m M .B../. a F G J L w I. a F. m E a l Q m w m 5" W l /G S I v J/I m n E O S 9 L R m Va PU 6 [W x D 2 J W a i C ms. so fl A\\\h\n$ b b /%7%/////// A w m m 5 A M v F u wmmfi n 3 2 w H w w 2 r United States Patent Ofl 3,366,824 Patented Jan. 30, 1968 13 Claims. 61. 313-197 The present invention relates to a low pressure spark system. More particularly, the invention relates to a low pressure spark system comprising a switching spark gap and a trigger spark gap for controlling the trigger spark gap.

A discharge circuit with low inductivity and rapid current build-up utilizes banks of capacitors with high charging power and voltage comprising individual capacitors having low charging power and voltage and connected in parallel with each other. Each of the individual capacitors usually has its own switch. The plurality of switches thus utilized must be operated simultaneously and must retain their dielectric stress after a great number of switching operations. The delay period of a switch, which is the time period between the switch control signal and the commencement of the application of the charging signal to the capacitor, comprises a substantially constant base time and a variable stray time. Since the constant base time may be compensated by an advance in timing, the stray time portion of the delay period remains to be compensated. This may be accomplished by utilizing switches with the shortest stray times and with great and adequate dielectric strength after a great number of operations.

Triggered high pressure spark gaps are widely used as the switches in capacitor discharge circuits. The working point of the triggered high pressure spark gap or spark path is to the right of the Paschen minimum of the ignition voltage or in the region wherein the ignition voltage increases almost proportionally with the product of the gas pressure of the spark gap and the distance between the electrodes, in accordance with Paschens law. A high pressure spark gap has a short build-up time and a short stray time if it is operated under favorable conditions. However, the electrodes of a high pressure spark gap burn otf rapidly, strong extraneous components or noise occur in a high pressure spark gap and a high pressure spark gap has a high inductance.

Ignitrons may be used as the switches in capacitor discarge circuits. The working point of the ignitron is to the left of the Paschen minimum of the ignition voltage or in the region wherein the ignition voltage increases with a small decrease in the product of the gas pressure of the spark gap and the distance between the electrodes. Ignitrons are suitable for voltages up to about 20 kilovolts and currents upto about 75 kiloarnperes. The self inductance of an ignitron is about nanohenries and its average life span is about 40,000 discharges. The life span of an ignitron is limited because the ignition resistance gradually decreases to less than one ohm and utlimately the ignitron can no longer ignite. Non-igniting ignitrons are not usable. Furthermore, an ignitron has a high inductance and a low current rating and these are disadvantages.

Low pressure spark gaps may be utilized to switch high currents at high voltages. A low pressure spark gap usually comprises a pair of substantially planar electrodes spaced a small distance such as, for example, approximately 2 cm., from each other. The electrodes are electrically insulated from each other by a gas under reduced pressure such as, for example, air at a pressure of 0.0 1 Torr or mm. of Hg, and by a tube or hollow cylinder of electrical insulating material which maintains the electrodes at the specified distance from each other and serves as the wall of the discharge vessel. Since the working point of a low pressure spark gap is to the left of the Paschen minimum, as with ignitrons, the product of the distance between the electrodes and the .gas pressure of the spark gap must be kept small enough so that the discharge is not triggered by the voltage to be transmitted or switched.

A low pressure spark gap may be ignited by an increase in the electron density in the area of the cathode. The electron density is increased by an auxiliary discharge which is known as the ignition or trigger discharge. An ignition or trigger spark gap or path is utilized to provde the trigger discharge. The stray portion of the delay period of the spark system comprising a switching spark gap and a trigger spark gap depends only upon the stray portion of the delay time of the trigger discharge when the trigger spark gap is at the cathode of the switching spark gap. A stray time of about 10 nanoseconds has been obtained in a trigger spark gap of known type comprising a pair of parallel tungsten wires of, for example, about 0.75 mm. diameter, and spaced at about 0.7 mm. from each other and embedded in a ceramic insulator with their ends extended over the surface of the ceramic insulator and across the cathode into the discharge space. A stray time of 10 nanoseconds, although it is small, is inadequate for use with one microsecond pulses to be transmitted or switched.

When the switching spark gap is ignited, the material of the electrodes evaporates. The quantity of vaporized material depends upon the type of electrode material and on the total charge at the spark gap. The electrode material vapor is precipitated on the electrodes and on the insulator and decreases the electrical resistance of the insulator and may destroy the dielectric strength of the spark gap after a few discharges.

The principal object of the present invention is to provide a new and useful low pressure spark system.

An object of the present invention is to provide a low pressure spark gap system having a high current rating.

Another object of the present invention is to provide a low pressure spark gap system having great dielectric strength at a high charge and high switching ability.

Another object of the present invention is to provide a low pressure spark gap system having a very small stray portion of the delay period.

Another object of the present invention is to provide a low pressure spark gap system in which only small erosions occur in the electrodes after prolonged high current discharges.

In accordance with the present invention, a low pressure spark gap system comprises a switching spark gap comprising a pair of substantially parallel, substantially planar electrode members spaced from each othera determined distance to provide a discharge space therebetween. Each of the electrode members comprises a material having a high vaporization point and good heat conductivity at the operating temperatures of the electrode members. An insulator positioned between and insulating the electrode members from each other comprises electrical insulating material having poor heat conductivity and adapted to react with vaporized material from the electrode members impinging thereon during discharge to produce material having no electrical conductivity. A trigger spark gap is positioned in operative proximity with the electrode members of the switching spark gap and comprises spaced electrodes and a semiconductor member positioned between the electrodes for guiding and sliding a trigger spark between the electrodes. The electrode members of the switching spark gap and the electrodes of the trigger spark gap comprise a penetration-bonded metal composition of tungsten penetrated by copper. The insulator comprises steatite and the semiconductor member comprises zinc cadmium oxide.

In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings, wherein:

FIG. 1 is a graphical presentation of the median stray portion of the delay period of a trigger spark gap of the present invention;

FIG. 2 is a graphical presentation of the delay period of the low pressure spark gap system of the present invention;

FIG. 3 is a view, partly in section, of an embodiment of a low pressure spark system of the present invention;

FIG. 4 is an enlarged view, partly in section, of the trigger spark gap of the embodiment of FIG. 3;

FIG. 5 is a view, partly in section, of another embodiment of the low pressure spark system of the present invention; and

FIG. 6 is a circuit diagram of an embodiment of the low pressure spark system of the present invention.

In PEG. 1, the abscissa represents the charge voltage Vc inkilovolts of the trigger spark gap and the ordinate represents the average or median stray portion is in nanoseconds of the delay period of the trigger spark gap of the present invention. The values of the stray time are determined by measurement and do not indicate the lowest stray times. The stray times may be decreased further by controlling the inductivity pulse source which applies the trigger voltage to the trigger spark gap.

FIG. 2 illustrates the delay period of the low pressure spark gap system of the present invention. In FIG. 2, the abscissa represents the delay time t in nanoseconds and the ordinate represents the ratio n/N in percent of unsuccessful discharges to the total number of discharges. FIG. 2 is a semilogarithmic presentation, as is customary in the representation of ignition delay time statistics. The distribution of delay periods provides a base or build-up time tb of about 47.8 nanoseconds and a median stray time is of 0.3 nanosecond. The trigger spark gap of the low pressure spark gap system has the same median stray time of 0.3 nanosecond.

The low pressure spark gap system of the present invention is readily disassembled and comprises coaxial spark gaps and substantially planar electrodes substantially parallel to each other, spaced a determined distance from each other and electrically insulated from each other by a hollow cylinder or ring of steatite. The discharge space contains air under low pressure such as, for example, 0.03 Torr. This prevents self-ignition of the switching spark gap when the voltage to be switched or transmitted is applied to the electrodes.

In PEG. 3, the axis of the low pressure spark system of the invention passes through the center of the rod 23 (FIG. 4) and the trigger spark gap 12 and the switching spark gap 1 to It) and 13 to 16 are coaxially positioned and symmetrical about such axis with the exception of the component 11. A discharge space 1 is enclosed by a hollow cylinder or ring 2 of steatite and by a cathode 3 and an anode 4. The cathode 3 has a substantially planar electrode disc 6 of tungsten and copper at its surface facing the anode 4. The anode 4 has a substantially planar electrode disc 5 of tungsten and copper at its surface facing the cathode 3. The trigger spark gap 12 is positioned in the cathode 3 and extends through the electrode surface disc 6 of said cathode.

The tungsten and copper of the electrode discs 5 and 6 are penetration-bonded in a composition which is highly fusible and resists burning off under light discharges. The quantity of metal vaporized from the electrodes is kept small by the electron discs 5 and 6 which produce only small erosions even at high current charges. The electrode 4 discs comprise a sintering metal. One component of the penetration-bonded composition such as, for example, the tungsten, is hi hly fusible and resists burning off. The pores of the tungsten are penetrated by the other component which may comprise, for example, copper, which has good heat conductivity.

The tungsten and copper of the electrode discs 5 and 6 need not be penetration-bonded throughout, but may be penetration-bonded only at their surfaces facing each other. The penetration-bonded composition may be compared to a sponge which is completely saturated with a liquid so that all of its pores are filled with the liquid. When the penetration-bonding is only partial, it may be compared to a sponge which is only partially submerged and then rapidly withdrawn from a liquid so that the greater part of the unsubmerged pores remain closed and dry whereas the submerged pores are filled with the liquid.

Molybdenum and rhenium, as well as tungsten, are suitable as one component of the electrode disc metal composition since they do not readily vaporize. Silver, as well as copper, is suitable as the other component of the electrode disc metal composition since it has good heat conductivity. The tungsten and copper combination is preferred due to its vaporiaztion cooling characteristic. In a specific temperature region, the copper begins to vaporize, while the tungsten does not vaporize. When the copper begins to vaporize, the vaporization consumes heat and thereby cools the tungsten. Since the tungsten and the copper are compounded, the cooling of the tungsten cools the electrode discs and the electrodes and the vaporiaztion ceases or is considerably slowed down. The cooling of the electrodes is accelerated by the good heat conductivity of the copper.

The ring 2 of electrically insulating material between the cathode 3 and the anode 4 of the switching spark gap has poor heat conductivity. The material utilized as the insulation ring 2 and the area of said ring bordering on the discharge space 1 are determined, in accordance with the present invention, to maintain the insulation resistance high, even after prolonged operation of the spark gap system. The insulation ring 2 functions as the discharge vessel, except for the top and bottom which are provided by the electrodes 3 and 4. The steatite material of which the insulation ring 2 is comprised is a ceramic. Porcelain and eucryptic ceramics, as well as steatite, are also suitable for the insulation ring 2. The poor heat conductivity of the insulation ring 2 causes it to have higher temperatures than the electrodes during discharge, so that metal evaporating from the electrode discs 5 and 6 and precipitated on said insulation ring is precipitated to a great extent back to said electrodes.

The principal function of the insulation ring 2 is to chemically bind the metal which vaporizes from the electrode discs 5 and 6 and impinges on said insulation ring to produce, at operational temperatures, materials which do not conduct electricity. The inner cylindrical surface of the insulation ring 2, which bounds the discharge space 1, is selected, for this purpose, to be of size such that even at the lowest charges, the heat transmitted in waves against said surface from the plasma is sufficient to produce a chemical reaction.

The heat to which the inner cylindrical surface of the insulation ring 2 is subjected increases the temperature of said inner cylindrical surface to the temperature necessary to produce a chemical reaction of the insulation material or steatite with the metal vapor impinging on said surface to produce non-electrically conductive material. The temperature should be just enough to soften the steatite to a vitreous condition to a surface layer depth of up to several microns. The temperatures required to produce such a chemical reaction are substantially dependent upon the softening regions of the insulation material utilized. If the insulation ring 2 comprises a metal silicate such as, for example, steatite, which is magnesium silicate, the first chemical reaction is the formation of metal silicates. This causes the dielectric strength or resistance of the insulation to remain unchanged after a multitude of switching operations or discharges.

If the insulation ring 2 is of cylindrical configuration and bounds the discharge space 1 with the top and bottom provided by the electrode discs 6 and 5, then the radius of the inner cylindrical surface of said insulation ring is of more importance than the axial length of said insulation ring. According to Paschens law, the axial length of the insulation ring 2 determines the type of discharge, or Whether a low pressure discharge may be produced or whether a high pressure discharge may be produced. An insulation ring 2 having a long axial length may just barely permit the production of a low pressure discharge and has the advantage of causing vaporized metal from the electrode surface discs 5 and 6 to impinge only on those portions of the inner cylindrical surface of said insulation ring which are adjacent said electrode surface discs. In this case, the central area of the inner cylindrical surface of the insulation ring 2, equidistant from both electrodes, remains unchanged.

The discharge space 1 is sealed gas-tight or vacuumtight by gaskets 7 and 8 which may comprise, for example, 0 ring type gaskets. The gaskets 7 and 8 are protected from the heat of the discharge space 1 by substantially annular lips or extending portions 9 and 10 of the insulation ring 2. The extending portions 9 and 10 are seated in corresponding annular grooves or channels formed in the cathode 3 and in the anode 4, respectively. A pump 31 of any suitable type for pumping air out of the discharge space 1 is operative via a conduit 11, which opens into said discharge space, and a gas inlet and pressure gage 32, which helps maintain a predetermined pressure in said discharge space.

The trigger spark gap 12 is described in detail with reference to FIG. 4. The trigger spark gap 12 produces a trigger spark when a suitable voltage pulse is supplied to it from a pulse generator connected to its electrodes. A suitable circuit for igniting the trigger spark gap is shown in FIG. 6. The discharge current is supplied to the cathode 3 coaxially via a flange or rim 13 formed and extending radially from the cylindrical portion 14 of the electrically conductive cathode. An electrical insulator 15 of substantially cylindrical configuration having a radially extending flange, in the manner of the cathode 3, is coaxially positioned between the cylindrical portion 14 of the cathode 3 and the steatite ring 2. This prevents the voltage to be switched or transmitted from arcing over in the area of the insulator ring 15, but restricts the discharge to the discharge space 1' and to between the cathode 3 and the anode 4; the voltage or signal to be switched or transmitted being applied to said anode and/ or cathode.

As shown in FIG. 4, the trigger spark gap 12 comprises 'a met al member 17 of hollow substantially cylindrical configuration. The member 17 has a shoulder 33 formed therein. An insulation member 18 of substantially cylindrical configuration is coaxially positioned in the member 17. The insulation member 18 may comprise, for example, steatite, and has a central axial bore formed therethrough. The insulation member 18 has a pair of spaced, opposite, substantially parallel, planar base ends such as, for example, a first base end 34 and a second 1 base end 35.

I A semiconductor disc 19 having heat resistance is coaxially aflixed to the first base end 34 of the insulation member 18 by any suitable means such as, for example, glue or cement. The electron density necessary for the ignition or triggering of the low pressure discharge is provided at the cathode by the trigger spark gap 12. The trigger spark gap 12 provides the required electron density via the semiconductor disc 19 which provides a slide, glide or guide for the trigger spark. The triggerspark has a small stray time. The trigger spark gap 12 is the ignition 6 electrode of the spark gap system and is positioned in and electrically insulated from the cathode 3 of the switching spark gap. The trigger spark is produced by the trigger spark gap 12 and is guided along the guide surface of the semiconductor disc 19 to the electrode surface disc 6 of said cathode. The semiconductor disc 19 may comprise, for example, Zinc-cadmium-oxide having high resistance to heat, or silicon carbide or similar types of semiconductor having high resistance to heat. The oxide semiconductor may comprise, for example, Zn Cd O, where x may be about 0.5. The minimum electrical resistance of the semiconductor disc 19 should be between one kilohm and one megohm or higher.

The utilization of a sliding spark path using a semiconductor guide or slide surface as the trigger spark path keeps the energy required for the build-up of the trigger spark to a very small value such as, for example, 1 to 2 milliwatts. Furthermore, ignition is possible at as low as one kilovolt and the average stray time of the spark delay period is less than one nanosecond at charge voltages higher than 5 kilovolts. The positioning of the trigger spark gap 12 in the cathode 3 of the switching spark gap provides a stray time of the delay period from the start of the trigger voltage to the increase of charge current in the switching spark gap which is equal to the stray time of the trigger spark. The stray time of the trigger spark delay period is not dependent upon the pressure at lower discharge air pressures than approximately 0.03 Torr. The stray time in a wide range of magnitudes of stray time is virtually independent of the resistance of the semiconductor material of the semiconductor disc 19, so that it is not effected by impurities which are precipitated upon the surface of said semiconductor disc when the switch spark gap discharges.

A threaded member 20 is threadedly coupled with the member 17, with a gasket or sealing ring 21 such as, for example, of 0 type, positioned between a step formed in the insulation member 18, said member and said threaded member. The threaded member 20 thus urges the outer surface of the semiconductor disc 19 against an end collar 22 of the member 17 via the insulation body 18. The end collar 22 of the member 17 comprises a compound of tungsten and copper, as do the electrode surface discs 5 and 6. The end collar 22 is of substantially annular configuration and tapers to a sharp annular edge or rim at its farthest extremity from the semiconductor disc 19.

An electrically conductive rod 23 is positioned in the central bore through the insulation member 18. One end of the rod 23 is at the semiconductor disc 19 and terminates in a substantially frustoconical head or a sharp conical head 24. The head 24 comprises a compound of tungsten and copper, as do the end collar 22 and the electrode surface discs 5 and 6. The head 24 thus has good heat conductivity and vaporizes only to a small extent during discharge. The semiconductor disc 19 has an axial bore formed therethrough and the rod 23 extends into said axial bore. The head 24 of the rod 23 is seated in the bore through the semiconductor disc 19 and a stepped portion of said head is urged against said semiconductor disc via the rod 23 and a nut 25 threadedly coupled to the other end of the rod. The nut 25 exerts pressure on the head 24 of the rod 23 against the semiconductor disc 19 because there is an end member 28 coaxially positioned on the rod and on the second base end 35 of the insulation member 18 and a washer 27 coaxially positioned on the rod between said end member and said nut. A sealing gasket or ring 26 of, for example, 0 type is positioned on the second base end 35 of the insulation member 18 between said insulation member and the end member 28.

The head 24 of the rod 23 and the end collar 22 comprise the electrodes of the trigger spark gap 12 and the semiconductor disc 12 provides the guide surface for the trigger spark. The member 17 is threaded in the area 29 of its end having the end collar 22 and the trigger spark gap is threadedly coupled in the cathode 3 of the switching spark gap via its threaded area 29. A sealing gasket or ring 16 such as, for example, of type, is positioned between a step formed in the cathode 3 and a step formed in the member 17 of the trigger spark gap 12.

In the embodiment of FIG. 5, two trigger spark gaps 12a and 12b are utilized instead of one as in the embodiment of FIG. 3. In the embodiment of FIG. 5, the conduit lla is coaxially positioned with the switching spark gap and the trigger spark gaps 12a and 1212 are diametrically positioned relative to said conduit. A plurality of trigger spark gaps may be utilized and are positioned equiradially and equiangularly from the conduit 11a and from each other, respectively. If a plurality of trigger spark gaps are utilized, they may be supplied from a common pulse source via separate lines.

Two or more trigger spark gaps are utilized to decrease erosion of the semiconductor disc 19, especially when the switching spark gap is subjected to high charge currents for prolonged periods of time. The positioning of the conduit lie coaxially with the switching spark gap permits said conduit to function as an expansion chamber for the discharge plasma if the plasma pressure becomes too high. This protects the discharge vessel from destruction by too high a plasma pressure. If a plurality of trigger spark gaps are utilized, as described, the discharge starts in the vicinity of the inner cylindrical surface of the insulation ring 2 and moves from the trigger spark gaps toward the axis of the switching spark gap.

The low pressure spark system of the present invention may be readily and tacilely disassembled to permit repair or replacement of the components thereof. It the high resistance and dielectric strength of the insulator ring 2 of the spark gap system are reduced due to an insufilcient thermal stress at its inner cylindrical surface, the electrical resistance value may be built-up by a plurality of forming discharges of high current intensity.

FIG. 6 shows a circuit utilizing low pressure spark gap systems of the present invention. In FIG. 6, each or" the spark gap systems 4t 41 and 42 comprises a low pressure spark gap system of the present invention. The spark gap system 40 is ignited or triggered by a trigger spark gap 43, the spark gap system 41 is ignited by a trigger spark gap 4 and the spark gap system 42 is ignited by a trigger spark gap 4-5. The trigger spark gaps 43, 44 and 45 are ignited by pulses from a pulse source 46, which may comprise any suitable source of pulses of suitable type.

A11 inductatnce 47 and a capacitor 50 are connected in series with the spark gap system 40, an inductance 48 and a capacitor 51 are connected in series with the spark gap system 41, and an inductance 49 and a capacitor 52 are connected in series with the spark gap system 42. The in ductances 47, 48 and 49 represent the inductances of the corresponding ones of the spark gap systems and each has an inductance value of from to uanohenries. Each of the capacitors 50, 51 and 52 has a capacitance value of, for example, 30 microfarads. The capacitors 50, 51 and 52 are charged by voltage from a high voltage source 56 prior to discharge or ignition of the trigger spark gaps. The capacitors 50, 5'1 and 52 are charged via resistors 55, 54 and 53, respectively. The voltage of the voltage source 56 may comprise, for example, kilovolts. The resistors 53, 54 and 55 have resistance values of about 100 kilohms.

The spark gap systems 40, 41 and 42 are connected in parallel with a load 57 which is to be switched into circuit via said spark gap systems. A shunt resistor 58 having a resistance value of, for example, one megacycle, is con nected across the load 57, especially when said load is a gas discharge vessel. The spark gap systems 4th, 41 and 42 are connected to a point 59 at ground potential. The load 57 may comprise the working coil or winding of magnetic apparatus such as, for example, magneform apparatus, which may function to discharge banks of capacitors having high charges and high voltages at specific determined times with small stray times.

In operation, the magneform or magnetic apparatus discharges the bank or banks of capacitors through the working coil or winding. A sufficiently large amount of electrical energy must be stored in the bank of capacitors to enable the solution of a specific forming problem. The switches in the discharge circuit must be able to conduct the maximum magnitude of the discharge current for a sufficiently large number of cycles of operation, and the low pressure spark gap system of the present invention is particularly suitable for such operation. The low pressure spark gap system of the present invention, when utilized in magneform apparatus, may function to connect the working coil into circuit or to short circuit the working coil. In the magneform opreation, it is desirable to short circuit the working coil the first time the charge current reaches the maximum magnitude. The current then decreases exponentially in the working coil. In this manner, the working coil current maintains a magnitude, higher than the minimum magnitude required for forming, for a longer time than is provided when the working coil current is periodically supplied.

The low pressure spark gap system of the present invention has also been successfully utilized to discharge large banks of capacitors and for short circuiting a load at maximum current. The low pressure spark gap system of the present invention may also be used as a load switch or a short circuit switch in a hydro spark operation, in which metal work members are formed by spark discharges ignited under water.

While the invention has been described by means of specific examples and in specific embodiments, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

I claim:

1. A low pressure spark system, comprising switching spark gap means comprising a pair of substantially parallel, substantially planar electrode members spaced from each other a determined distance to provide a discharge space therebetween, each of said electrode members comprising a material having a high vaporization point and good heat conductivity at the operating temperatures of said electrode members, and insulating means positioned between and insulating said electrode members from each other, said insulating means comprising electrical insulating material having poor heat conductivity and adapted to react with vaporized material from said electrode members impinging thereon during discharge to produce material having no electrical conductivity; and

trigger spark gap means positioned in operative proximity with the electrode members of said switching spark gap means.

2. A low pressure spark gap system as claimed in claim 1, wherein the electrode members of said switching spark gap means comprise a penetration-bonded metal composition.

3. A low pressure spark gap system as claimed in claim 1, wherein the electrode members of said switching spark gap means comprise a penetration-bonded metal composition of tungsten penetrated by copper.

4. A low pressure spark gap system as claimed in claim 1, wherein the insulating means of said switching spark gap means comprises steatite.

5. A low pressure spark gap system as claimed in claim 1, wherein the insulating means of said switching spark gap means is of substantially cylindrical configuration having an inner cylindrical surface between the electrode members of said switching spark gap means and bounding the discharge space therebetween, said inner cylindrical surface having an area and shape of dimensions determined to provide suflicient heat at said inner cylin- 9 drical surface during discharge to produce said material having no electrical conductivity.

6. A low pressure spark gap system as claimed in claim 1, wherein said trigger spark gap means is positioned in one of the electrode members of said switching spark gap means.

7. A low pressure spark gap system as claimed in claim 1, wherein said trigger spark gap means comprises spaced electrodes and a semiconductor member positioned between said electrodes for guiding and sliding a trigger spark between said electrodes.

8. A low pressure spark gap system as claimed in claim 7, wherein the semiconductor number of said trigger spark gap means comprises oxide semiconductor material.

9. A low pressure spark gap system as claimed in claim 7, wherein the semiconductor member of said trigger spark gap means comprises zinc cadmium oxide.

10. A low pressure spark gap system as claimed in claim 7, wherein the electrode members of said switching spark gap means and the electrodes of said trigger spark gap means comprise a penetration-bonded metal composition.

11. A low pressure spark gap system as claimed in claim 7, wherein the electrode members of said switching spark gap means and the electrodes of said trigger spark gap means comprise a penetration-bonded metal composition of tungsten penetrated by copper.

12. A low pressure spark gap system as claimed in claim 1, wherein said low pressure spark gap system has an axis and wherein said trigger spark gap means comprises a single trigger spark gap coaxially positioned with said switching spark gap means.

13. A low pressure spark gap system as claimed in claim 1, wherein said low pressure spark gap system has an axis and wherein said trigger spark gap means comprises a plurality of trigger spark gaps equiradially positioned relative to said axis and equiangularly positioned relative to each other.

References Cited UNITED STATES PATENTS 2,300,931 11/1942 Kalischer et al. 313-325 X 2,553,444 5/1951 Schlesman 313217 X 2,720,474 10/1955 Myers 313-217 X 3,328,623 6/1967 Hale et al. 3133l1 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner. 

1. A LOW PRESSURE SPARK SYSTEM, COMPRISING SWITCHING SPARK GAP MEANS COMPRISING A PAIR OF SUBSTANTIALLY PARALLEL, SUBSTANTIALLY PLANAR ELECTRODE MEMBERS SPACED FROM EACH OTHER A DETERMINED DISTANCE TO PROVIDE A DISCHARGE SPACE THEREBETWEEN, EACH OF SAID ELECTRODE MEMBERS COMPRISING A MATERIAL HAVING A HIGH VAPORIZATION POINT AND GOOD HEAT CONDUCTIVITY AT THE OPERATING TEMPERATURES OF SAID ELECTRODE MEMBERS, AND INSULATING MEANS POSITIONED BETWEEN AND INSULATING SAID ELECTRODE MEMBERS FROM EACH OTHER, SAID INSULATING MEANS COMPRISING ELECTRICAL INSULATING MATERIAL HAVING POOR HEAT CONDUCTIVITY AND ADAPTED TO REACT WITH VAPORIZED MATERIAL FROM SAID ELECTRODE MEMBERS IMPINGING THEREON DURING DISCHARGE TO PRODUCE MATERIAL HAVING NO ELECTRICAL CONDUCTIVITY; AND TRIGGER SPARK GAP MEANS POSITIONED IN OPERATIVE PROXIMITY WITH THE ELECTRODE MEMBER OF SAID SWITCHING SPARK GAP MEANS. 